This invention relates to mineral recovery processes which comprise a main separation stage and a waste sedimentation stage.
In the main separation stage the mineral, for instance phosphate clay, is slurried with water and is separated into an enriched fraction and a dilute aqueous clay waste. This separation stage may involve one or more separation processes and may involve recycling of enriched material for further separation.
The waste sedimentation stage involves sedimenting and evaporating the dilute aqueous clay waste in one or more settling lagoons to provide a substantially solid clay sediment and supernatant. The supernatant is recycled in order to provide as much as is reasonably possible of the water which is to be used in the main separation stage. If the aqueous clay waste contains valuable mineral, the waste may be subjected to treatments to allow recovery of this mineral before the final sedimentation of the clay.
The aqueous clay wastes formed in phosphate recovery and other such processes are sometimes referred to as slimes and are subjected to lagoon sedimentation because they are not susceptible to normal dewatering processes but, instead, settle only very slowly.
There have been numerous proposals in the literature to try to accelerate the sedimentation by flocculating the clay waste, and there have been proposals to improve the structure of the substantially solid clay sediment by adding sands or other materials to the clay waste. Examples of disclosures of such mineral recovery processes utilizing flocculants are U.S. Pat. Nos. 3,418,237, 3,622,087, 3,707,523, 4,194,969, 4,224,149, 4,251,363, 4,265,770, 4,342,653, 4,555,346, 4,690,752, 1,446,185 and 5,688,404, which are each incorporated herein by reference.
Despite the numerous proposals to use flocculants, in practice it has been found that their use frequently is not cost effective. Even when flocculant is used to promote sedimentation and the provision of a supernatant which can be recycled, the quality of the supernatant tends to be rather poor because the supernatant tends to be contaminated with unflocculated clay particles.
In particular, the main separation stage often includes the use of treatment chemicals such as flocculation agents or, especially, flotation agents and the efficiency of their use is decreased (and thus the required dosages are increased) when the water which is used in the flotation or other procedures during the separation stage contains suspended clay particles.
In order to minimize contamination and lack of clarity of the supernatant, it would be desirable to conduct the sedimentation under conditions which provide a considerable depth of sedimenting waste, so as to allow for a deep layer of supernatant above the sediment, thereby permitting supernatant to be drawn off at a height which is as far above the lower sedimented material as is possible. Unfortunately it is difficult to provide for this in lagoons as they tend normally to be relatively shallow. In particular, the problem becomes more acute as the lagoons become filled, over the years, with an increasing depth of substantially solid clay sediment.
A further problem arises from the fact that it is necessary to make optimum use of lagoon areas because of the undesirability of creating new lagoons. Accordingly there is an increasing tendency to need to continue using lagoons until it is impossible to deposit any more solid clay sediment in them, and so there is an increasing tendency to want to use lagoons which are substantially full and are too shallow for useful sedimentation. There is an increasing need to utilize lagoon areas more efficiently.
It is of course known in other processes to utilize sedimentation columns, for instance tubular metal tanks, which are constructed above ground level. Provided such a column has sufficient height it will eventually allow for the formation of a useful depth of supernatant. Unfortunately the volume of aqueous clay wastes which are generated in phosphate and other mineral recovery processes can be so large that it is impracticable even to contemplate the provision of column separating tanks of this type.
It is also, of course, well known to extend the life of a lagoon by digging the solid clay sediment out of it, but this is labor intensive and does not provide any direct solution to the need to conduct the recovery process efficiently and to give a good quality supernatant.